Monitoring systems of this type have been described, for example, in U.S. Pat. No. 4,193,023 issued in the names of two of us, Robert Buck and Gerd Marhofer, and in various earlier patents cited therein. A system of this nature usually includes an electronic switch, such as a thyristor, which is triggered to energize a load for signaling the occurrence of the event to be monitored. The trigger signal may be derived from the output of an oscillator whose amplitude changes, e.g. in response to the approach of a ferromagnetic element, as a result of such occurrence.
U.S. Pat. No. 4,135,124, in the name of Robert Buck, discloses the use of an Exclusive-OR gate as a means for enabling the selective emission of a high or a low output signal upon an increase or a decrease of the oscillator voltage whereby an electronic switch such as a thyristor will fire when the element being monitored either approaches or departs from the vicinity of the motion detector; with its anode tied to a supply of raw-rectified alternating voltage, the thyristor is continuously retriggered in the presence of a high voltage at the output of the Ex-OR gate so as to draw an increased load current. The Ex-OR gate, therefore, acts as a selective inverter for the control signal emitted by the motion detector.
Since the load current in such a system is either "on" or "off", depending on the conductive or nonconductive state of the electronic switch, a malfunction resulting in a prolonged absence of such load current may go undiscovered by being wrongly interpreted as either the occurrence or the nonoccurrence of the event to be detected. It is therefore desirable to provide means in a system of this nature for indicating the existence of a malfunction to an operator. For this purpose it has already been proposed to provide the system with two outputs carrying antivalent signals so that either one or the other output will conduct in any event. Under some circumstances, however, a malfunction could disable the detector while still keeping one of these outputs energized.
In copending application Ser. No. 415,442 filed Sept. 7, 1982 by three of us, namely Robert Buck, Gerd Marhofer and Hans-Dieter Gesthuysen, there has been disclosed an improved malfunction indicator in a monitoring system of this character which more or less excludes the possibility that a failure of the detector or a breakdown in its output circuit be overlooked. According to that disclosure, the system comprises a source of a train of recurrent pulses along with modulating means connected to that source and to an output lead of the associated detector for shifting the phase of these pulses whenever the binary load-control signal present on that lead assumes a certain value, the modulating means being connected to supervisory means for emitting an alarm indication in response to a malfunction-induced interruption of the pulse train emitted by the source.
Since the pulse train--whether phase-shifted or not--will always reach the supervisory means during normal operation, its absence will be a sure sign that a malfunction has occurred. The supervisory means may include for this purpose a pulse integrator generating a finite test signal during normal operation.
While the pulse train could be phase-modulated in various ways to indicate the occurrence or nonoccurrence of the condition monitored, e.g. the approach or departure of a ferromagnetic element in a machine tool or the like, the preferred design of the modulating means is in the form of an anticoincidence gate of the Exclusive-OR type which, in acting as a selective inverter as known per se from the aforementioned U.S. Pat. No. 4,135,124, transmits the incoming pulses with or without phase inversion. When the pulse source is a square-wave generator with a duty ratio of substantially 50%, the magnitude of the test signal emitted by the aforementioned pulse integrator will remain virtually unchanged in the event of a phase reversal.
As further described in that copending application, the pulse train issuing from the anticoincidence gate is fed--preferably after amplification--to a comparator in the form of a second Exclusive-OR gate forming part of the supervisory means. Another input of this second Ex-OR gate receives a reference wave from the pulse generator so that the two pulse trains appearing at that gate are either in phase or in phase opposition, depending on the output signal of the detector. The output voltage of the second Ex-OR gate will be very low (theoretically zero) in the case of cophasal pulses but will be relatively high in the presence of antiphasal pulses; this voltage, therefore, can serve as a useful d-c signal turning a load on and off. If, however, a malfunction interrupts the pulse train coming from the first Ex-OR gate, this output voltage will be intermittent and therefore of reduced but non-zero average amplitude; the simultaneous disappearance of the test signal will further indicate the off-normal condition.
While the malfunction indicator of the copending application operates satisfactorily in checking on the overall integrity of the monitoring system, it does not specifically verify the correct functioning of the motion detector proper, such as the aforementioned oscillator.